| dc.description.abstract | Thermoelectric (TE) materials provide an environment-friendly approach to convert waste heat into electricity. The liquid-like superionic behavior of copper selenides and their ultra-low thermal conductivity have attracted interest as potential thermoelectric material. However, to reach practical applications, the thermoelectric performance of copper selenides requires enhancement. In this thesis, the literature on TE materials is reviewed, the routes for synthesizing different copper selenide materials are discussed, and the strategies for enhancing the thermoelectric performance through alloying, doping, nanostructuring, and nanocomposite formation are discussed. Moreover, the application of graphene as an excellent filler to improve the performance of TE materials is analyzed.
We developed a solution-based method for synthesizing pure copper selenide and its graphene oxide (GO) nanocomposites. Two types of GO prepared following the Tours (GOT) and the Hummers (GOH) methods, leading to a different distribution of oxygen functionality, were used. Detailed analysis of the chemical composition, structural morphology, mechanical and thermal properties, and the thermal conductivity of pure and nanocomposite thermoelectric materials was carried out.
The structure of the synthesized materials is characterized using XRD and XPS that confirmed the cubic crystal structure of Cu1.8Se with the absence of impurities. The SEM analysis confirmed the sheet-like morphology with homogenous size distribution synthesized material. The nanocrystal size of Cu1.8Se was ~42 nm based on the Warren-Averbach model. The addition of graphene did not have a significant effect on the grain size of the nanopowders. However, the Hummer’s GO composites showed a smaller grain size than Tour’s GO composites. The thermal transitions and thermal stability were analyzed using DSC and TGA. The incorporation of graphene increased both the mechanical and thermoelectric properties. The Cu1.8Se and its nanocomposites showed poor thermal stability due to the sublimation of selenium at higher temperatures. This was reflected in poor TE performance and the inability to hot-press the nanocomposites due to melting.
Nonetheless, thermal annealing at 600°C for 10 hours has improved the thermal stability of the pure and nanocomposite samples. The annealed sample also showed enhanced electrical conductivity, Seebeck coefficient, and power factor. The power factor of the pure sample increased from 0.04 mW/mK² to 0.56 mW/mK² at 300 °C after the thermal treatment.
Our results show that incorporating graphene into copper selenide is an encouraging method to improve the thermoelectric performance of the materials, with the improvement of mechanical properties that may help in device fabrication for commercial applications. Furthermore, the thermal treatment of the nanopowders under inert conditions has drastically enhanced the material's thermal and TE properties. | |
